FIGS. 6 and 7 illustrate a tandem master cylinder according to the state of the art. This master cylinder 300, only the portion of which near secondary cylinder 330 is shown, consists of body 310 with bore hole 320 in which is housed secondary piston 330 and, if need be, a primary piston, not shown, if master cylinder 300 is a tandem master cylinder. Actuation of the master cylinder occurs in the direction of arrow AF. In bore hole 320, secondary piston 330 delimits pressure chamber 340 connected to the secondary brake system. Pressure chamber 340 is fed with brake fluid from supply chamber 350. Pressure chamber 340 is separated from supply chamber 350 by resupply seal 353 placed in groove 351 of body 310. On the other side of supply chamber 350, impermeability with the exterior is provided by seal 380 housed in groove 352 of body 310 and pressing against secondary piston 330. Secondary piston 330 consists of end wall 331, wherein rear face 331b receives the thrust rod from the servobrake and face 331b receives telescoping rod 370, formed of two parts and kept separated by spring 373. This rod 370 is connected to the primary piston.
Secondary piston 330 is a machined aluminum part with a cylindrical skirt 332 equipped with end wall 331, one face of which, 331b, is equipped with a frustoconical housing to receive the end of thrust rod 370 connected to primary piston 132 and whose other face, 331a, serves as a support for return spring 373. The forward edge of skirt 332 has a frustoconical surface 334 traversed by a crown of holes 333. Secondary piston 330 delimits secondary pressure chamber 340 of secondary circuit C2 in the master cylinder and cooperates with resupply seal 353 housed in groove 351 machined in bore hole 320. This groove 351 is located beyond supply chamber 350, connected to the hydraulic fluid supply line; seal 353, cooperating with the exterior surface of secondary piston 330, thereby ensures a seal between supply chamber 350 and pressure chamber 340. However, when secondary piston 330 is in rest position, its front edge is positioned slightly forward of supply chamber 350 so that brake fluid can pass beneath the seal through holes 333 in the front of skirt 332 of secondary piston 330 and reach pressure chamber 340.
Such a piston and master cylinder are described in German Published Patent Application No. 10 2006 000341.
FIG. 7 is an axial cutaway of secondary piston 330, which is known, illustrating the shape of skirt 332 with its frustoconical front portion 334 and crown of holes 333, as well as the shape of end wall 331, with face 331a and a boss for centering spring 373 (FIG. 6), and other face 331b with a frustoconical housing to accommodate telescoping rod 370 connected to the principal piston.
FIG. 6A, which is a very schematic cutaway, illustrates the rest position of resupply seal 353 in its groove 351 of body 310 of the master cylinder, as well as supply chamber 350 and skirt 332 of secondary piston 330, with the crown of holes here represented by a single hole, 333, placed beneath seal 353. An arrow indicates the passage of hydraulic fluid during the resupply phase, between supply chamber 350, hole 333, the interior of secondary piston 330 and, in front of it, pressure chamber 340. This figure emphasizes the need for the conical shape, for the flow rate in rest position during ESP operation of the brake system is a function of the diameter of the holes as well as the space between the piston and body 310 of the master cylinder.
This secondary piston 330 has a certain number of manufacturing as well as functional drawbacks:
Manufacturing is relatively expensive because of the crown of holes 333 that needs to be made in the front edge of skirt 332 and because of the frustoconical shape 334 that is given to the edge of the skirt. Additionally, holes 333 realized in the edge of the skirt often have burrs that cause premature wear to seal 353, which cooperates with secondary piston 330.